1. Technical Field of the Invention
The present invention relates to an electromagnetic attraction type magnetic bearing which always sets the control current of one of electromagnets which face each other to zero, and control method thereof.
2. Description of the Related Art
An electromagnetic attraction type magnetic bearing has advantages that friction or wear does not occur and lubrication is not required, by virtue of non-contact supporting, and can be utilized as bearings of a turbo compressor, an ultralow temperature rotary machine, a turbo charger, a flywheel, and the like.
As this electromagnetic attraction type magnetic bearing, the inventors of the invention has invented magnetic bearings composed only of electromagnets, and have already filed an application (Patent Documents 1 and 2).
These magnetic bearings are those in which a current is applied to a pair of electromagnets which face each other with the float therebetween by a two-input nonlinear control method in a direction in which the displacement from the center position of the float is offset, and the float is maintained at a neutral position.
The “active control electromagnetic attraction type magnetic bearing” of Patent Document 1 aims at reducing a power loss, generation of heat of the electromagnets, and resistance against a rotational motion of the float.
Therefore, in the active control electromagnetic attraction type magnetic bearing, as shown in FIG. 1, a two-input nonlinear control method is used as a control method of applying currents i1 and i2 to a pair of electromagnets 52 and 53 which face each other with the float 51 therebetween in a direction in which the displacement from a neutral position O of the float 51 is offset, and maintaining the float 51 at the neutral position O.
The “electromagnetic attraction type magnetic bearing and its nonlinear control method” of Patent Document 2 aims at exerting a damping force even when return to a balance point from deviation is made, thereby making it possible to shorten convergence time, and completely satisfying the asymptotically stable condition of Lyapunov which is the sufficient condition of the control method.
Therefore, as shown in FIG. 2, this device includes at least a pair of electromagnets arranged to face each other, a float 61 arranged between the electromagnets and held at the middle position thereof, a sensor 62 for detecting the displacement of the float 61 from a balance position, and/or the speed thereof, and a controller 63 for controlling the electromagnets. In a case where the float 61 returns to the balance position, a control current is supplied to both the electromagnets which face each other by the controller 63, and thereby, one electromagnet is made to act as a tension spring, and the other electromagnet is made to act as a damper.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-165164    Patent Document 2: Japanese Patent Application Laid-Open No. 2002-61646